A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Part of the qualification of a lithographic apparatus relates to the system focus qualification. In existing lithographic apparatus, focus qualification is usually performed on qualification wafers especially prepared for focus qualification, as these allow to provide reproducible test results.
It is, however, also desirable to perform focus test during operational use of the lithographic apparatus, especially on production wafers. Production wafers may have already been exposed to one or more production steps, and as a result already comprise product features. Crosstalk from the focus test system and these features can influence the results of a focus test. In particular, results are obtained from a focus test, but the reliability of the results may be questionable.